This invention relates to an anti-lock brake system (ABS) for a four-wheel drive vehicle, and more particularly to an ABS system in which torque transfer between the front and rear of the vehicle is detected and alleviated.
In four wheel drive vehicles, the front wheels are mechanically coupled to the rear wheels through a transfer case, so that the sum of the front wheel speeds is made equal to the sum of the rear wheel speeds. When the vehicle has an anti-lock brake system (ABS) that is configured to control slip independently at the front and rear wheels, the sum of the front and rear wheel speeds becomes unequal during ABS braking. This results in an undesired torque transfer between the front and rear wheels which can degrade the performance of the ABS control.
It has been proposed to detect the existence of torque transfer based on wheel slip or the brake pressure ratio between the front and rear wheels, but these techniques either provide only limited information or require expensive pressure sensors. In some systems, the pressure may be estimated in lieu of using pressure sensors, but many ABS controls utilize on/off solenoid control, rendering pressure estimation impractical. One example of a known torque transfer detection and control is anonymously disclosed in the July, 1995 issue of Research Disclosure.
The present invention is directed to an improved four-wheel drive anti-lock brake control in which the existence of torque transfer between the front and rear wheels of the vehicle is detected based on differences in the duration of ABS control used at each of the independently controlled wheels. Such differences are used to determine a ratio of rear-to-front (or front-to-rear) ABS control. If the ratio indicates the presence of torque transfer, the wheel causing the transfer is identified and its brake pressure released as required to alleviate the detected torque transfer, so long as the release will not cause a change in vehicle deceleration.